The shape-memory effect of shape-memory alloys occurs due to the transition from the beta-phase at high temperatures to the thermoelastic martensite phase at low temperatures. The effect is either irreversible or reversible. Applications which use the irreversible shape-memory effect are found in connectors and couplings, and those which utilize the reversible effect are in window openers, heat-actuated water sprinklers and heat-actuated safety switches, as well as thermo-driven apparatus such as heat engines.
Furthermore, shape-memory alloys can be used in spectacles frames or vacuum seal packings since they have super-elastic effects and will, even placed under a strain of several to ten-odd percent, return to the original shape upon removal of the stress. Shape-memory alloys also have vibration-proofing effects and find use in the manufacture of gears and other various machine parts that require vibration- and sound-proofing.
Many shape-memory alloys have been proposed and some of them are currently used on a commercial basis. One of the commercial shape-memory alloys is based on copper and contains 10-35% Zn and 1-12% Al, with the balance being Cu and incidental impurities (the percent being on a weight basis and will be so hereunder). This alloy has excellent shape-memory effects and is getting particular attention of researchers.
Despite its excellent shape-memory properties, the above described Cu-Zn-Al alloy has two serious problems: firstly, it undergoes intercrystalline cracking under relatively small internal stresses caused either by restraining the displacement that will otherwise occur or by application of a load; secondly, the alloy has a reduced resistance to heat cycles in that the behavior of reversible transformation between the martensite and beta-phases changes to such an extent that the amount of potential shape restoration is decreased.